Electrostatic marking system with a load stabilized power supply

ABSTRACT

A circuit adapted to provide marking and reverse potentials for an electrostatic recorder utilizing a single power supply which may be of poor regulation, the circuit including switching means and ballast means arranged so that the load on the power supply is substantially constant during both the marking and erase operations so that stability of the output of the power supply is achieved.

United States Patent [191 Smith et al. Aug. 20, 1974 [5 ELECTROSTATIC MARKING SYSTEM 2,263,322 3;; gamma 22%: 5 l rown, r.

LOAD STABILIZED POWER 3,757,036 9/1973 Libbet et a 346/74 ES [75] Inventors: John W. Smith, Whitestone, N.Y.; Prima Ex ry ammer mcent P. Canney John Long Oakland N Assistant ExaminerJay P. Lucas [73] Assignee: Muirhead, lnc., Mountainside, NJ. Attorney, g Firm-Edwal'd Connors [22] Filed: Feb. 5, 1973 7 ABS ACT [21] Appl. No.: 329,646 1 TR A circuit adapted to provide marking and reverse potentials for an electrostatic recorder utilizing a single [2%] (S1. ower supply which may be of poor regulation, the 1 46 4 E S SB circuit including switchrng means and ballast means [58] new of Search /7 5 8, arranged so that the load on the power supply is substantially constant during both the marking and erase operations so that stability of the output of the power [5 6] References Cited Supply is achieved.

UNITED STATES PATENTS 12/1969 Gold .f. 346/74 ES 3 Claims, 1 Drawing Figure ELECTROSTATIC MARKING SYSTEM WITH A LOAD STABILIZED POWER SUPPLY BACKGROUND OF THE INVENTION distant point and reproduced in a recorder is scanned an elemental area of the copy at a time, and line-byline. Light from an elemental area of the copy is converted to an electric signal for transmission to the distant point. At the receiving or recording end, the received signals are reproduced by scanning, for example, an electrolytic recording medium line-by-line and producing upon the successive elements of each line a mark. The density of themark is controlled by the electric signal so that the mark is proportionate in density to the light from the elemental area of the copy. The characteristics of an electrolytic recording medium is such that the density of the mark is generally proportionate to the magnitude of the marking current.

In using an electrostatic recording medium certain problems are presented in thatit is inherently a black and white system and as such is practically incapable of reproducing gray scale or half tone pictures. Therefore, in electrostatic recording it has been necessary to resort to a different method which produces a picture similar to the picture produced by the screening process used in making printed pictures. In this process images are formed by the use of a plurality of dots of various sizes. Thus a lighter tone is achieved by a plurality of small sized spaced dots while a darker tone is achieved by making larger sized dots. Another variable is produced by the movement of the recorder stylus as it moves in its line-by-line scanning motion.

The size and shape of the electrostatic image formed on the recording medium depends upon thesize and shape of the electrodes, the magnitude and polarity of the voltage applied, the electric field intensity in the gap between the printing stylus and the backing electrode, and the movement of the electrodes with respect to the marking medium,

In the case where high resolution is a desirable feature as in the transmission of half tone'copy the expansion of the deposited charge pattern due to the stylus dimensions has been a problem. Such expansion of the deposited charge pattern due to the stylus dimensions may result in the overlapping of adjacent deposited 7 charge patterns in response to closely spaced actuating pulses. This action severely limits the resolution of-the system. A method of overcoming the effect of the expansion of the deposited charges has been disclosed in US. Pat. No. 3,484,792 to Murray J. Gold. This patent teaches the method and apparatus for marking an electrostatic recording medium in which a first marking pulse of one polarity is applied to the stylus during its scanning motion, and applying a second erase pulse of opposite polarity while partof the charge transferred by the stylus in response to the record pulse is still beneath the stylus. By this method substantially all charges of the marking polarity are removed from beneath the stylus. Thus, because of'the movement of the stylus, the marking charge left on the recording me- 2 cent-shaped image. The size of the image maydepend upon the design characteristics of the system so that the image may be considerably smaller than the electrode from which it was made.

It is known that best results are achieved when the required marking potential is greater than the required erasing potential. For example, a common value for the marking potential is about 650 volts, while the erasing potential is about 450 volts. To maintain uniform optical density of the recorded mark it is essential that the marking potential be stabilized. A problem is presented in stabilization of the relatively high potential because the power supply must alternately provide high voltage marking and erase potentials. Heretofore the practice has been to independently stabilize the power supply against load and voltage changes. Inorder to do this expensive components have been required.

SUMMARY OF THE INVENTION Thepresent invention aims to overcome the difficulties and disadvantages of prior electrostatic marking systems by providing such a system in which the power supply need not be stabilized.

In accordance with the invention there is provided a marking system in which the marking load and the erase loads are made approximately equal and the loading on the power supply is continuous. This is accomplished by providing a single power supply and usingthecontrol signal for actuating switching means for reversing the potential applied to the stylus and platen so that the load upon the power supply is kept constant. In order to provide the same loading for an erase potential of lesser voltage than the marking potential a ballast load is included for the erase stage.

The invention is advantageous in that there is automatically a good stabilization of the power supply although the power supply itself maybe poorly regulated;

BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 is a schematic drawing of a marking system in accordance with the invention.

, DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing there is shown an electrostatic marking system in accordance with the invention. A suitable analogue facsimile signal is applied at 10 and passed through resistor 11 to base 12 of transistor 13. Emitter 14 is grounded and collector 15 is connected through a resistor 16 to a power supply 17.

Preferably the analogue facsimile signal is in the form of a series of generally triangular waveforms as taught dium, when a circular electrode is used, is a small cresin co-pending application Ser. No. 329,649 filed Feb. 5, 1973 by John H. Long and titled Apparatusfor Half Tone Marking of Electrostatic Paper. As described in the application the triangular waveforms are constant in amplitude but varied with respect to a threshold in proportion to the transmitted image. The transistor 13 acts as a combination threshold detector andmarking transistor in converting the analogue signal to a series of output pulses representative thereof, the threshold being the base-emitter conduction threshold of the transistor 13. Altemately the analogue signal supplied to the terminal may be of the conventional type.

The collector output of the transistor 13 is applied through a resistor to base 21 of a transistor 22 having its emitter 23 grounded and its collector 24 connected through a resistor 25 to a power supply 26. The output of the collector 24 is connected to a stylus 27 in marking position with a platen 28. An electrostatic recording paper or medium 29 is adapted to be moved between the stylus 27 and the platen 28 for marking.

The output of collector 15 is also connected through a resistor 31 to base 32 of a transistor 33 having its emitter 34 grounded. Collector 35 of the transistor 33 is connected through a resistor 36 to the power supply 17. Additionally, the collector 35 is connected through a resistor 37 to base 40 of a transistor 42 having its emitter 42 grounded, and its collector 43 connected through a resistor 44 to the power supply 26. A ballast resistor 45, if required, is connected across the emitter 42 and the collector 43. The output of the emitter 43 is connected to the platen 28.

In the operation of the circuit an analogue signal of either triangular waveform or conventional type is applied to the marking transistor 13. When the analogue signal is positive the transistor 14 turns on and its collector 15 goes to zero because of the drop in the resistor 16. While the transistor 13 is conducting the transistor 22 is cut off so that positive 650 volts is applied to the stylus 27. When the transistor 13 is conducting the transistor 33 is cutoff and the transistor 42 is saturated so the platen 28 is at ground potential. The opposite case applies while the transistor 13 is not conducting and the transistor 22 grounds the stylus 27 and a positive potential is applied to the platen 28 to effect a negative erase potential to the stylus 27.

As stated above a preferred marking potential is about 650 volts while the erase potential is about 450 volts. The ballast resistor 45 drops the potential at the collector 43 to provide the desired erase potential of 450 volts.

The high voltage transistors 22 and 40 require appreciable collector-emitter current flow during conduction to achieve short switching times (typically 15 m.a. or more). This load places a burden on the voltage regulation characteristics of the 650 volt power supply. Since either one of the transistors 22 or 40 is conducting at all times, and the two transistors are never conducting simultaneously it is possible to balance their loads so that the load on the power supply is constant.

All of the transistors used in a working embodiment of the invention were NPNs. Transistors 22 and 40 were Delco DTS 704. Transistors l3 and 33 were Texas Instrument 3704.

The circuit constants were as follows:

Resistor 11 2.2 K ohms Resistor 16 510 ohms Resistor 20 5 l0 ohms Resistor 25 25000 ohms Resistor 31 5.1 K ohms Resistor 36 510 ohms Resistor 37 510 ohms Resistor 44 25000 ohms Resistor 45 55 K ohms While the invention has been described and illustrated with reference to a specific embodiment thereof, it will be understood that other embodiments may be resorted to without departing from the invention. Therefore, the form of the invention set out above should be considered as illustrative and not as limiting the scope of the following claims.

We claim:

1. ln an'electrostatic marking system for a facsimile recorder including a stylus and a platen operatively positioned to receive an electrostatic recording medium therebetween, the marking system including the power supply, switching means, control means responsive to a facsimile signal for actuating said switching means, and circuit means providing one polarity of said power supply to said stylus and the other polarity of said power supply to said platen and upon actuation of said switching means for reversing the polarity of said stylus and said platen, the improvement comprising potential dropping means connected into said circuit means so that in one switched condition the potential across said stylus and platen is lower thereacross than in the other switched condition, and ballast means connected into said circuit with said potential dropping means whereby the load on said power supply is at least practically equalized under both switched conditions.

2. An electrostatic marking system for a facsimile recording according to claim 1 in which said control means includes means responsive to a facsimile signal waveform variable in amplitude.

3. An electrostatic marking system for a facsimile recorder according to claim 2 in which said means responsive to a facsimile signal waveform variable in amplitude is a transistor supplied by the facsimile signal I through its base. 

1. In an electrostatic marking system for a facsimile recorder including a stylus and a platen operatively positioned to receive an electrostatic recording medium therebetween, the marking system including the power supply, switching means, control means responsive to a facsimile signal for actuating said switching means, and circuit means providing one polarity of said power supply to said stylus and the other polarity of said power supply to said platen and upon actuation of said switching means for reversing the polarity of said stylus and said platen, the improvement comprising potential dropping means connected into said circuit means so that in one switched condition the potential across said stylus and platen is lower thereacross than in the other switched condition, and ballast means connected into said circuit with said potential dropping means whereby the load on said power supply is at least practically equalized under both switched conditions.
 2. An electrostatic marking system for a facsimile recording according to claim 1 in which said control means includes means responsive to a facsimile signal waveform variable in amplitude.
 3. An electrostatic marking system for a facsimile recorder according to claim 2 in which said means responsive to a facsimile signal waveform variable in amplitude is a transistor supplied by the facsimile signal through its base. 